The main objective of this PPG is to identify novel influenza immunogens based on conserved regions of the HA and immunization protocols resulting in broad protective humoral immunity against multiple strains and subtypes of influenza A viruses. It has been known for many years that antibodies directed against specific regions of the HA protein of influenza virus have potent virus neutralizing activities, and play an important role in protection against infection after natural exposure or vaccination. Also for many years it was assumed that protection induced by HA antibodies was highly specific for single virus strains, as, in general, virus infection and/or vaccination only protect against infection with the same strain of influenza virus. This established dogma in the influenza virus field has been shattered by the recent discovery and generation of multiple monoclonal antibodies directed against conserved regions in the HA molecule that have broad neutralizing activities. In general, this type of cross-reacting antibodies is present only at low frequencies in immunized individuals, most likely due to the immunodominance of strain specific epitopes. Our collaborators in this PPG, Peter Palese (Project 1) and Patrick Wilson (Monoclonal Antibody Technology Core) have developed techniques for the generation and identification of these broadly neutralizing antibodies in mice and in humans. In order to take advantage of these breakthroughs we will, in collaboration with Project 1 and the Technology Core, investigate the potency of the different antibodies against different influenza virus strains in a mouse model (Aim 1). Aim 1 studies will inform to Project 1 on specific antibodies with potent and broad efficacy in vivo for further characterization by Project 1 on epitope specificity. In addition. Aim 1 will identify specific antibodies with a good potential to be developed as therapeutic tools for the treatment of severe influenza. Furthermore, to explore vaccination strategies based on HA-based immunogens recognized by broadly neutralizing monoclonal antibodies, we will use the immunogens developed by Project 1 (PI: Palese) and immunization protocols developed by Project 2 (PI: Ahmed) to investigate the ability of these immunogens to generate protective immune responses in two animal models, the mouse and the ferret (Aim 2), and the mechanism of antibody-mediated neutralization and protection (Aim 3). The information generated by Aims 2 and 3 will provide the basis for potential further development into novel